The pion-muon death cycle - a double anniversary

15 October 1997  

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The life and death of a pion and a muon are captured in a streamer chamber, where gas glows brightly along the tracks of the particles. Pions and muons were first seen in cosmic rays, the rain of high-energy particles from outer space. Muons were discovered 60 years ago, by Carl Anderson and Seth Neddermeyer at Caltech. Muons live for about 2.2 microseconds, and often survive to ground level, before changing into electrons and invisible neutrinos. In 1947, ten years after the muon discovery, Cecil Powell's group at Bristol University discovered that the muons are produced by other particles - pions - which live for only a few hundredths of a microsecond. In this image, pions fly out from a collision in the streamer chamber. One of the pions makes the looping track to the right, before it decays into a muon, which then curls anticlockwise four times, and eventually changes into an electron which moves off towards the upper right.

Credit: CERN Photo

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